NIR spectroscopy is a well-known spectroscopic technique that looks specifically at the absorptions of infra-red radiation with frequencies of above 4000 cm−1. NIR spectroscopy can be used to measure the intensity of the overtones of the molecular vibrations in a molecule, containing carbon-hydrogen, oxygen-hydrogen, and nitrogen-hydrogen bonds. The carbon-hydrogen (C—H) absorption bands are typically useful for mixtures of organic compounds. Different types of C—H bonds, e.g., aromatic, aliphatic, and olefinic hydrocarbons, absorb light at different characteristic frequencies. The magnitude of the absorption band in the spectra is proportional to the number of C—H bonds present in the sample. Hence, NIR spectroscopy is often used to obtain a fingerprint of a sample and by empirically correlating the said fingerprint the intrinsic properties of the sample may also be known.
The NIR region between 780 nanometers (nm) and 2500 nm (12800 to 4000 cm−1) is an area of great interest and contains a large amount of molecular information in the form of combinations and overtones from polyatomic vibrations. Mathematical techniques are essential to utilize this information and to calculate the desired properties. U.S. Pat. Nos. 5,490,085; 5,452,232; and 5,475,612, for example, describe the use of NIR for determining octane number, yields and/or properties of a product of a chemical process or separation process from analysis on the feeds to that process, and yields and/or properties of a product of a blending operation again from analysis on the feed thereto.
NIR spectroscopy can be applied to crude oils and other hydrocarbon refinery streams. WO 00/039561 and WO 03/048759, for example, both describe application of NIR to crude oil analysis.
The analysis of crude oil samples, for example, can be performed by generating chemometric models correlating spectral data from “standard” (i.e. characterised) crude oil samples with the known properties of the samples, and subsequently applying said models to the spectra of “unknown” samples to characterise the properties thereof.
“Chemometrics” is the application of mathematical and statistical techniques to the analysis of complex data, and hence “chemometric model” means a model generated from application of such techniques in correlating the spectral data from a sample with properties of the sample and cell pathlength. The chemometric model determines the relationship between the spectral data and the cell pathlength as it would for the chemical and/or physical properties (via eigenvectors of a covariance matrix).
The generation of the chemometric model can be done using any one of a variety of techniques/mathematical and statistical techniques, as described, for example, in Principal Component Analysis, I. T. Jolliffe, Springer-Verlag, New York, 1986; D. M. Halland and E. V. Thomas, Anal. Chem., 60, 1202 (1988) or K. R. Beebe and B. R. Kowalski, Anal. Chem., 59, 1007A (1987).
The analysis of samples such as crude oils is typically done using a transmission cell into which the sample is introduced. The cells typically have a relatively short pathlength so that a reasonable signal is transmitted through the cell. However, such cells require cleaning when used with substances such as crude oils. This is by no means a trivial task when using fixed (solid) cells, so demountable cells are the preferred option. Demountable cells may be disassembled, cleaned and then reassembled again for re-use.
The problem faced with demountable cells, however, is that during the disassembly and reassembly the pathlength of the cell may change. With cells that have a relatively short pathlength, even small changes in the pathlength can have significant effects on the spectra obtained. For example where spectral data from “standard” (i.e. characterised) crude oil samples is being measured for generation of a suitable chemometric model correlating various properties of the crude oil samples with the spectral data, the variations in cell pathlength can have significant detrimental effects on the model obtained.
By convention there are three main types of spectroscopic probes that can be utilised within the aforementioned NIR transmission cells; the transflection (transmission reflection) probe, ATR (attenuated total reflectance probe) and the DRIFTS (Diffuse Reflectance Infrared Fourier Transform Spectroscopy) probe. The transflection probe comprises an NIR probe containing two optical fibres, normally silica, such that light is passed down one fibre which is then projected through a lens/window at the lower end of the probe and reflected—via a mirror—back through the window into the return fibre. A gap between the window and the mirror—the sample support—allows the sample of interest to enter into the light beam and thus an absorption spectrum is obtained.
The DRIFTS probe is similar to the former however there is no mirror present in the probe per se to reflect the light back into the return fibre and as such is normally used to collect spectra from solid samples.
Unfortunately, whilst assembling the transmission cell in order to characterise a sample, as air is present in the optical path, it generates significant inaccuracies in the data obtained.